Fix crash caused by accessing finishing task

In our design, there are two existing recents task at the same time. If
start new recents while finishing previous recents, there are two subtasks under recents root task, one is newly created and the other
is finishing subtask. Try to get activity from finishing subtask would
be null and cause NPE when manipulate null activity.

Append non-finishing check could ensure we get activity record from
newly created subtask instead of finishing subtask.

Test: atest CtsWindowManagerDeviceTestCases

Change-Id: I3fe8357e57c4f1afe578c696e8371874be0ed45e